These medications paralyze all skeletal muscle, including the diaphragm muscle.
NMBAs, especially the steroid-based rocuronium and vecuronium are associated with reports of prolonged recovery and myopathy. For this reason it is critical to use the lowest possible NMBA dose for the shortest possible duration.
Continuous infusions of neuromuscular blocking agents are usually given to critically ill patients for 1 of 2 reasons:
1. To facilitate mechanical ventilation
2. To prevent movement
To facilitate mechanical ventilation
When given to facilitate mechanical ventilation, NMBAs prevent dysynchrony with the ventilator, stop spontaneous respiration and muscle movement, and improve gas exchange.
To prevent movement
When given to prevent movement, NMBAs are used in patients with tetanus, shivering from therapeutic hypothermia, or certain surgical procedures if movement would affect the anastamosis. Because shivering from therapeutic hypothermia usually does not occur once the target temperature is reached, I prefer to use bolus doses of NMBAs rather than continuous infusions for these patients.
Sedation while on NMBAs
It is critical that all staff understand that NMBAs do not provide sedation or analgesia. Patients must be sedated continuously for the duration of paralysis to avoid an “awake & paralyzed” scenario.
When orienting new critical care nurses, I advocate a dual monitoring strategy for NMBAs:
1. Clinical goal assessment
2. Train-of-four assessment
First, the clinical goal must be assessed. Most often this goal is “compliance with the ventilator”. If the clinical goal is not being met, the NMBA infusion should be titrated upwards.
If the clinical goal is met, then the train-of-four should be assessed.
The train-of-four is used to assess the depth of paralysis from a NMBA. A peripheral nerve stimulator is used to deliver 4 pulses of electricity, and the twitch from each pulse is counted. The depth of paralysis correlates to the train-of-four as follows:
When 4/4 twitches are seen, paralysis is 0-75%.
When 3/4 twitches are seen, paralysis is at least 75%.
When 2/4 twitches are seen, paralysis is 80%.
When 1/4 twitch is seen, paralysis is 90%.
When 0/4 twitches are seen, paralysis is 100%.
Assuming the clinical goal is met, the NMBA infusion should be titrated to a goal of 1/4 or 2/4 twitches. If the clinical goal is met and 0/4 twitches are present, the infusion should be briefly held and restarted at a lower rate when 1/4 twitches return.
If the clinical goal is met and the patient has 3/4 or 4/4 twitches, no change needs to be made to the rate of infusion.
Determining the electricity “dose” on the peripheral nerve stimulator
A common pitfall I’ve seen in NMBA monitoring involves incorrect use of the peripheral nerve stimulator for monitoring.
Most peripheral nerve stimulators deliver 10-70 mA impulses and are adjusted in 10 mA increments.
In an ideal world, the amount of mA that gives 4 strong twitches would be determined before the NMBA infusion is started. This is done by increasing the mA dose in a step-wise fashion starting at the lowest setting. Once 4 strong twitches are seen, attempt at one setting higher; if the twitches are not any stronger then move back down to the previous setting.
Here is an example:
1. 4 strong twitches are seen at 20 mA.
2. 30 mA is attempted and the twitches are no stronger than at 20 mA.
3. The patient’s train-of-four is always assessed at 20 mA.
If the patient is paralyzed before a baseline mA dose can be established, choose an arbitrary mA dose of 20-40 mA, and always assess the patient at this mA level.
Once the NMBA infusion is going, if 0/4 twitches occurs don’t increase the mA dose. This will lead to overuse of the NMBA. Instead look at the clinical goal and make adjustments accordingly.
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